1. Field of the Invention
This invention relates to polymer dispersed liquid crystal films, and to methods of forming the same by controlling the conditions of photopolymerization to control the properties of the film.
2. Description of the Related Art
A new type of material referred to as a polymer dispersed liquid crystal (PDLC) film has been developed recently with applications to various electro-optical switching devices. The material is described in U.S. Pat. No. 4,688,900, issued Aug. 25, 1987 to Joseph W. Doane et al., and assigned to Kent State University. The PDLC film consists of droplets or bubbles of liquid crystal molecules (LC) dispersed in a clear or light transmitting, flexible plastic sheet or film. The PDLC film is capable of being thermally, electrically, magnetically or electromagnetically addressed, causing the material to be reversibly switched between a light scattering mode and a light transmissive mode. The material is optically responsive to strain, so that under tension it acts as a polarizer that transmits one component of plane polarized light while scattering the other component.
The PDLC is prepared by dissolving liquid crystal molecules in an uncured monomer resin, and then curing or polymerizing the resin so that bubbles or droplets of liquid crystal spontaneously form and are uniformly dispersed throughout the matrix. The exact "curing" method is not described, but it is known that PDLCs may be cured by a photopolymerization process of exposure to ultraviolet light, as described by Vaz, Smith and Montgomery in Mol. Cryst. Liq. Cryst., 146. pp. 1-15 (1987). The patent also suggests that epoxy resins which are curable by UV radiation are useful. The resulting liquid crystal droplets are described in the patent as being of uniform size and spacing, and having a diameter ranging upward from about 0.2 microns depending primarily upon the curing procedure and the specific materials used.
The PDLC is described as exhibiting a reversible, high contrast optical response from an opaque scattering mode to a clear light transmission mode when either the temperature of the material is changed, or an electric field is applied. The material is optically responsive to strain, whereby under tension it acts to polarize incident light.
When cured under normal conditions, a relatively high operating voltage of about 60-100 volts is required to maintain the film in a light transmissive state, and a relatively high threshold voltage of perhaps about 40 volts is also required to initiate the transition from light scattering to light transmission. These high voltages require the use of relatively expensive electronic drivers.
The Doane patent also describes a curing process in which the film is cured in the presence of a magnetic field of sufficient strength to cause the liquid crystal molecules in the droplets to align in the direction of the field. When the curing process is complete, the alignment becomes permanent and persists upon removal of the applied field. By curing while applying the magnetic field directed in the plane of the film, the resultant film can act as a switchable polarizer by off-state scattering of light polarized in the alignment direction, and by the absence of scattering when an electrical field is applied in the transverse direction across the film.
The Doane patent also describes a PDLC film cured in the presence of a transverse electrical field to give a film which is transparent in the off-state, and which becomes scattering when exposed to a high intensity electromagnetic source. Such a film acts as a nonlinear device which is normally transparent and can be used as a protective coating against a high intensity laser beam.
An alternate method is described in the patent for aligning the liquid crystal molecules during cure, involving the application of a compressive strain to the film. With this approach the liquid crystal molecules are aligned parallel to the surface of the film, rather than perpendicular, and switch from a light scattering to a transmissive mode when an AC voltage of sufficient strength is applied.
A related type of liquid crystal film is described in U.S. Pat. No. 4,435,047 to J. L. Fergason. In this patent a liquid crystal is dispersed in an aqueous polyvinyl alcohol slurry, and cast and dried to a thin film. The resulting encapsulated liquid crystal film, which is produced by Taliq Corporation, has relatively high operating and threshold voltages, similar to the unaligned PDLC films of Doane et al.
A related patent application by J. D. Margerum et al., "Polymer Dispersed Liquid Crystal Film Devices, and Method of Forming the Same", was filed Jan. 5, 1988 under Ser. No. 141,033, and assigned to Hughes Aircraft Company, the assignee of the present application. This application describes the control of various PDLC film characteristics, such as threshold and operating voltages, contrast ratio, absolute transmission levels, transmission wavelengths and optical response times, by a corresponding control over various photopolymerization conditions, including the UV intensity, temperature, LC concentration, initiator concentration and UV wavelength. However, for a given LC bubble size the required operating and threshold voltages may still be undesirably high.